Method for fabricating MOS transistor

ABSTRACT

A method for fabricating metal-oxide semiconductor (MOS) transistor is disclosed. The method includes the steps of: providing a semiconductor substrate having a silicide thereon; performing a first rapid thermal process to drive-in platinum from a surface of the silicide into the silicide; and removing un-reacted platinum in the first rapid thermal process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for fabricating MOS transistor, andmore particularly, to a method of using rapid thermal process todrive-in platinum from surface of silicide layer into the silicidelayer.

2. Description of the Prior Art

Field effect transistors are important electronic devices in thefabrication of integrated circuits, and as the size of the semiconductordevice becomes smaller and smaller, the fabrication of the transistorsalso improves and is constantly enhanced for fabricating transistorswith smaller sizes and higher quality.

In the conventional method of fabricating transistors, a gate structureis first formed on a substrate, and a lightly doped drain (LDD) isformed on the two corresponding sides of the gate structure. Next, aspacer is formed on the sidewall of the gate structure and an ionimplantation process is performed to form a source/drain region withinthe substrate by utilizing the gate structure and spacer as a mask. Inorder to incorporate the gate, source, and drain into the circuit,contact plugs are often utilized for interconnection purposes, in whichthe contact plugs are composed of conducting metals such as tungsten andcopper. Nevertheless, the interconnection between the contact plugs andthe silicon material of the gate structure and the source/drain regionis usually poor, hence a silicide material is often formed over thesurface of the gate structure and the source/drain region to improve theohmic contact between the contact plugs and the gate structure and thesource/drain region.

Today, the process known as self-aligned silicide (salicide) process hasbeen widely utilized to fabricate silicide materials, in which asource/drain region is first formed, a metal layer comprised of cobalt,titanium, or nickel is disposed on the source/drain region and the gatestructure, and a first rapid thermal process (RTP) is performed to reactthe metal layer with the silicon contained within the gate structure andthe source/drain region to form a silicide layer. After using a sulfuricacid-hydrogen peroxide mixture (SPM) cleaning to remove un-reactivenickel from the first rapid thermal process, a second RTP is conductedto reduce the sheet resistance of the silicide layer.

Unfortunately, the cleaning process conducted between the aforementionedfirst and second RTP typically removes un-reacted metal entirely. Evenif some metals remained after the cleaning process, they are preferablyconcentrated relative to the surface of the silicide and could notpenetrate into the interface between the silicide and the semiconductorsubstrate. This causes a junction leakage between the PN junction of thesource/drain region and the silicon substrate and the silicide beingformed and results in a piping phenomenon. Hence, how to effectivelyresolve this issue has become an important task.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide a method offabricating MOS transistor to resolve the above issue of junctionleakage during a salicide process.

According to a preferred embodiment of the present invention, a methodfor fabricating metal-oxide semiconductor (MOS) transistor is disclosed.The method includes the steps of: providing a semiconductor substratehaving a silicide thereon; performing a first rapid thermal process todrive-in platinum from a surface of the silicide into the silicide; andremoving un-reacted platinum in the first rapid thermal process.

Another aspect of the present invention provides a method forfabricating MOS transistor. The method includes the steps of: providinga semiconductor substrate, wherein the semiconductor substrate comprisesa source/drain region thereon; forming a Ni-Pt layer and a barrier layeron the source/drain region; performing a first rapid thermal process toform part of the Ni-Pt layer and the source/drain region into asilicide; removing un-reacted nickel and barrier layer from the firstthermal process; performing a second rapid thermal process to drive-inplatinum from a surface of the silicide into the silicide; removingun-reacted platinum from the second rapid thermal process; andperforming a third rapid thermal process for lowering the resistance ofthe silicide.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 illustrate a method for fabricating a MOS transistor accordingto a preferred embodiment of the present invention.

FIGS. 4 is a flow chart diagram illustrating the process after theaforementioned first RTP according to the preferred embodiment of thepresent invention.

FIGS. 5-8 are perspective diagrams illustrating fabrication processconducted after the second RTP.

DETAILED DESCRIPTION

Referring to FIGS. 1-3, FIGS. 1-3 illustrate a method for fabricating aMOS transistor according to a preferred embodiment of the presentinvention. As shown in FIG. 1, a semiconductor substrate 100, such as awafer or a silicon-on-insulator (SOI) substrate is provided. Preferably,the semiconductor substrate 100 may include silicon structures such asgate electrode, source/drain regions, isolation regions, word lines,diodes, fuses, or resistors depending on different product demands andfabrication processes. According to the preferred embodiment of thepresent invention, a gate structure 106, source/drain region 112, andisolating region 128 of a MOS transistor are exemplified in thisembodiment. As shown in FIG. 1, the gate structure 106 includes a gatedielectric layer 102 and gate electrode 104. The gate dielectric layer102 is preferably composed of insulating material such as siliconnitrides, oxides, oxynitrides, or metal oxides, and the gate conductivelayer 104 is composed of conductive material such as doped polysilicon,silicides, metal compounds or other metals.

Next, a lightly doped ion implantation process is performed by using thegate electrode 104 as mask to implant dopants into the semiconductorsubstrate 100 adjacent to two sides of the gate conductive layer 104 forforming a source/drain extension or a lightly doped source/drain 110.The implanted dopants are preferably selected according to the type ofMOS transistor being fabricated. For instance, n-type dopants includingphosphorus or arsenic would be implanted for fabricating a NMOStransistor, whereas p-type dopants including boron would be used for aPMOS transistor. Additionally, a spacer (not shown) could be selectivelyformed on the sidewall of the gate structure 106 through hot oxidationprior to the formation of the source/drain extension or the lightlydoped source/drain 110. By doing so, this selectively formed spacer andthe gate electrode 104 could be using as a mask during the lightly dopedion implantation process.

A liner 107 composed of silicon oxide and one or more spacer 108composed of silicon nitride compound are selectively formed on thesidewall of the gate structure 106, in which the liner 107 and thespacer 108 could be composed of any dielectric material. Next, a heavilydoped ion implantation is performed by using the gate electrode 104 andthe spacer 108 as mask to implant heavy dopants into the semiconductorsubstrate 100 for forming a source/drain region 112. Similar to the ionimplantation conducted for the aforementioned lightly doped source/drain110, dopants implanted for a NMOS transistor would include phosphorus orarsenic, whereas dopants implanted for a PMOS transistor would includeboron. Next, a thermal annealing process is performed by using atemperature between 1000° C. to 1050° C. to activate the dopants withinthe semiconductor substrate 100 and repair the damage of the crystallattice structure of the semiconductor substrate 100 caused during theion implantation process.

In addition to the aforementioned process, the order for fabricating thespacer, the lightly doped source/drain and the source/drain region couldbe adjusted according to the demand of the product, which are all withinthe scope of the present invention. For instance, in one embodiment, oneor more spacer could be formed, the source/drain is formed thereafter,and after removing the spacer or the outer most layer of the spacer, ionimplantation is conducted to form the lightly doped drain region. Inanother embodiment, two recesses could be formed in the substrate withrespect to two sides of the gate structure prior to the formation of thesource drain region, and an epitaxial layer could be grown throughselective epitaxial growth process in the two recesses thereafter. Theepitaxial layer is preferably composed of material suitable for NMOStransistor, such as SiC, or material suitable for PMOS transistor, suchas SiGe.

Next, a salicide process is conducted to form silicide layers. As shownin FIG. 2, a pre-clean could be selectively conducted to remove nativeoxide or other impurities from the surface of the gate structure 106 andthe source/drain region 112, and then a thin film deposition process isconducted to deposit a metal layer 114 with approximately 150˜200Angstroms and a barrier layer 116 with approximately 100˜200 Angstromscomposed of TiN on the surface of the gate structure 106 and thesource/drain region 112. The metal layer 114 preferably comprises afirst metal comprising platinum (Pt), nickel (Ni), cobalt (Co), titanium(Ti) or alloys of the aforementioned metals used to form silicide and, asecond metal comprising Pt, Co, palladium (Pd), manganese (Mn), tantalum(Ta), ruthenium (Ru) or alloys of the aforementioned metals in a lowconcentration. The second metal is added with a concentration of 3-8%(wt %) and is preferably used to improve a thermal stability of thesalicide and prevent the salicide from agglomeration which increasescontact resistance and junction leakage. In this embodiment, the firstmetal is Ni and the second metal is Pt. However, in a modification ofthe preferred embodiment, the first metal is not limited to Ni, but canbe Co or Pt; and, the second metal used to improve thermal stability isnot limited to Pt, but can also be Pd, Mo, Ta, or Ru. Next, a firstrapid thermal process (RTP) is performed to react the metal layer 114with silicon in the gate structure 106 and the source/drain 112 and toform transitional silicides 118 while defining the thickness of thesilicides 118 simultaneously. These processes are well known to thoseskilled in the art and further detailed description is therefore omittedhere for brevity.

Referring to FIGS. 4-7, FIGS. 4 is a flow chart diagram illustrating theprocess after the aforementioned first RTP according to the preferredembodiment of the present invention, and FIGS. 5-7 are perspectivediagrams illustrating fabrication process corresponding FIG. 4. As shownin the figures presented, step 132 is first carried out to perform thefirst RTP for reacting the Ni—Pt metal layer with silicon to formintegrated silicides. In this embodiment, the temperature of the firstRTP is lower than 300° C., preferably between 240° C. to 290° C., andthe duration of the first rapid thermal process is between 30 seconds to120 seconds, and preferably at 45 seconds.

In step 134, a sulfuric acid-hydrogen peroxide mixture (SPM) cleaningprocess is performed to remove the barrier layer 116 composed of TiN andun-reacted nickel metal from the first RTP, as shown in FIG. 5.Preferably, some of un-reacted platinum metal 117 would remain on thesurface of the transitional silicides 118. In this embodiment, theduration of the SPM cleaning process is between 500 seconds to 700seconds, and preferably at 600 seconds. The temperature of the SPMcleaning process is preferably at 95° C., and the volume percent ofsulfuric acid to hydrogen peroxide in SPM is preferably 800:200.

In step 136, a second RTP is performed to drive-in the remainingplatinum metal from the surface of the silicide 118 into the silicide118, such as to the boundary between the silicide 118 and thesource/drain region 112. By using the platinum as barrier, the presentinvention could prevent nickel atoms from penetrating into the substrate100 to cause junction leakage and piping phenomenon. In this embodiment,the fabrication parameters of the second RTP is similar to the ones usedin the first RTP, such that the temperature of the second RTP is between240° C. to 290° C., and the duration of the second RTP is between 30seconds to 120 seconds.

In step 138, a hydrochloric acid-hydrogen peroxide mixture (HPM)cleaning process is conducted to remove un-reactive and remainingplatinum from the second RTP. It should be noted that this cleaningprocess preferably removes the platinum remained on the surface of thetransitional silicide 118 while not affecting any of the platinum beingdriven into the silicide 118 and the overall thickness of thetransitional silicide 118 is remained the same. In this embodiment, theduration of the HPM cleaning process is between 210 seconds to 410seconds, and preferably at 310 seconds. The temperature of the HPMcleaning process is preferably at 50° C., and the volume percent ofhydrochloric acid to hydrogen peroxide in HPM is preferably 800:600.Thereafter, another cleaning process could be carried out selectively byusing SPM to remove remaining barrier layer 116 and un-reacted nickelmetal.

Next, an ammonia hydrogen peroxide mixture (APM) cleaning process couldbe selectively conducted to remove remaining particles from the surfaceof the semiconductor substrate 100. In this embodiment, the duration ofthe APM cleaning process is between 20 seconds to 220 seconds, andpreferably at 120 seconds. The temperature of the APM cleaning processis preferably at 60° C., and the volume percent of ammonia, hydrogenperoxide, and water in APM is preferably 60:120:2400.

In step 140, a third RTP is conducted to transform the transitionalsilicide 118 into a silicide layer with lower sheet resistance. In thisembodiment, the third RTP is preferably a spike anneal process, and thetemperature of this process is greater than 300° C., and preferablybetween 400° C. to 500° C.

After the third RTP is conducted, as shown in FIG. 7, a contact etchstop layer (CESL) 120 is formed on top of the silicide layer 118 as noextra cleaning process is performed between the second RTP and theformation of the CESL 120. The material of the CESL 120 is preferablydependent upon the nature of the NMOS or PMOS transistor, such that theCESL 120 could either be a CESL 120 with tensile stress or compressivestress.

As shown in FIG. 8, an interlayer dielectric layer 122 composed ofoxides is deposited on the semiconductor substrate 100 to cover the CESL120. The interlayer dielectric layer 122 could be composed of nitrides,oxides, carbides, low-k dielectric material or combination thereof.

Next, a contact plug fabrication is performed by using a patternedphotoresist (not shown) as mask to etch through the interlayerdielectric layer 122 and the CESL 120 for forming a plurality of contactopenings 124 exposing the silicide layer 118 on top of the gatestructure 106 and the source/drain region 112. A metal composed oftungsten, TiN or other conductive material is then deposited in thecontact openings 124 for forming a plurality of contact plugs 126electrically connecting the silicide layer 118. This completes theformation of a MOS transistor with silicides.

It should be noted that before the contact plug fabrication, theaforementioned process could be integrated with a gate-last for high-kfirst or gate-last for high-k last approach for forming metal gatetransistors with high-k dielectric layer. Moreover, the process of thepresent invention could also be applied to salicide process in othersemiconductor device, such as non-planar transistors including Fin-FETs.

As the cleaning process conducted between the two RTP steps inconventional approach typically removes platinum entirely, or leavingonly part of platinum on the surface of the silicide without enteringthe interface region between the silicide and the silicon substrate, thepresent invention preferably conducts a RTP process after the removal ofbarrier layer and nickel from the silicide to drive-in platinum into aninterface region between the bottom of silicide layer and semiconductorsubstrate, removes un-reacted platinum from the surface of the silicide,and then performs another RTP to transform transitional silicide into asilicide layer with lower resistance. By applying the aforementioneddrive-in RTP process, the present invention could use the platinumbetween silicide and substrate as a barrier to prevent nickel atoms fromthe silicide entering the substrate to cause junction leakage andpiping, thereby ensuring the quality of silicide being formed.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A method for fabricating metal-oxidesemiconductor (MOS) transistor, comprising: providing a semiconductorsubstrate, wherein the semiconductor substrate comprises a source/drainregion thereon; forming a Ni—Pt layer and a barrier layer on thesource/drain region; performing a first rapid thermal process to formpart of the Ni—Pt layer and the source/drain region into a silicide;removing un-reacted nickel and barrier layer from the first thermalprocess; performing a second rapid thermal process to drive-in platinumfrom a surface of the silicide into the silicide; removing un-reactedplatinum from the second rapid thermal process; and performing a thirdrapid thermal process for lowering the resistance of the silicide. 2.The method of claim 1, wherein the temperature of the first rapidthermal process and the second thermal process is between 240° C. to290° C.
 3. The method of claim 1, wherein the duration of the firstrapid thermal process and the second rapid thermal process is between 30seconds to 120 seconds.
 4. The method of claim 1, wherein thetemperature of the third rapid thermal process is higher than 300° C. 5.The method of claim 1, wherein the barrier layer comprises TiN.
 6. Themethod of claim 1, further comprising: using a sulfuric acid-hydrogenperoxide mixture (SPM) for removing un-reacted nickel and the barrierlayer from the first rapid thermal process; and using a hydrochloricacid-hydrogen peroxide mixture (HPM) for removing un-reacted platinumfrom the second rapid thermal process.